Saturday, 15 August 2015

Weekend plot: ATLAS weighs in on Higgs to Tau Mu

After a long summer hiatus, here is a simple warm-up plot:

It displays the results of ATLAS and CMS searches for h→τμ decays, together with their naive combination. The LHC collaborations have already observed Higgs boson decays into two 2 τ leptons, and should be able to pinpoint h→μμ in Run-2. However, h→τμ decays (and lepton flavor violation in general) are forbidden in the Standard Model, therefore a detection would be an evidence for exciting new physics around the corner. Last summer, CMS came up with their 8 TeV result showing a 2.4 sigma hint of the signal. Most likely, this is just another entry in the long list of statistical fluctuations in the LHC run-1 data. Nevertheless, the CMS result is quite intriguing, especially in connection with the LHCb hints of lepton flavor violation in B-meson decays. Therefore, we have been waiting impatiently for a word from ATLAS. ATLAS is taking his time, but finally they published the first chunk of the result based on hadronic tau decays. Unfortunately, it is very inconclusive. It shows a small 1 sigma upward fluctuation, hence it does not kill the CMS hint. At the same time, the combined significance of the h→τμ signal increases only marginally, up to 2.6 sigma.

So, we are still in a limbo. In the near future, ATLAS should reveal the 8 TeV h→τμ measurement with leptonic tau decays. This may clarify the situation, as the fully leptonic channel is more sensitive (at least, this is the case in the CMS analysis). But it is possible that for the final clarification we'll have to wait 2 more years, once enough 13 TeV data is analyzed.

16 comments:

Anonymous
said...

If the BR gets established at 1% level then would it also mean TeV scale new particles that LHC should find? Or would it be 10TeV scale new particles out of LHC's reach? Would New Physics (NP) contributions to this BR be generally order ~ (Weak scale/NP scale) or (Weak/NP)^2? Thanks!

It scales as BR~(Weak/NP)^2, but with a large coefficient in front because the SM Higgs width is very suppressed. To explain BR~1% you need NP~multi TeV, so there is no strict guarantee of new particles at the LHC. However, most realistic models give additional suppression of BR by the small SM lepton masses, and then one needs NP~weak scale.

How easy is it to distinguish H-->TT-->T-mu from H-->T-mu directly at the LHC?

The tau mean lifetime isn't really all that long (2.9*10^-13 seconds) and a certain percentage of T in any set of 2 are going to take a second decay step reasonable quickly (certainly in less than the mean lifetime on average, an even faster for the soonest conversion infrequent enough to match the observed T-mu decay rate).

Hi Jester, I saw something like this on Motl blog but I'd just feel a bit more comfy getting it from a cold headed pro than a SUSY enthusiast (it's just me no offence Josef). In any case would it take a lot to compile a list of still standing SM anomalies from LHC with their current status? Just curious, thanks!

Anon, lepton flavor violation in neutrino oscillations is a tiny effect, which can be due to new physics at a very high scale, even as high as 10^15 GeV. That effect indeed does induce h > tau mu decays, but the branching fraction is so small that it can never be observed. On the other hand, observing lepton flavor violation in Higgs decays would point to new physics around 1 TeV or below.

Andrew, h>tau tau is indeed one of the backgrounds, but not the worst one (e.g. Z > tau tau is larger). There are some angular variables that help to isolate the signal, but in general the separation is difficult. At the end of the day, they just look for an excess on top of known backgrounds around the tau-mu invariant mass of 125 GeV.

RBS, I think Lubos' list is more or less complete. However, one should remember that the number of deviations in ATLAS and CMS data what one expects just from background fluctuations, given the number of searches performed. So any single anomaly in this list is most likely just a fluctuation of the SM background (including the ones I wrote about in this blog).

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Résonaances is a particle physics blog from Paris. It's about the latest news and gossips in particle physics and astrophysics. The posts are often spiced with sarcasm, irony, and a sick sense of humor. The goal is to make you laugh; if it makes you think too, that's entirely on your own responsibility...